Advertisement

Advertisement

Science: Fiery future for planet Earth

By
JEFF HECHT in
BOSTON

The good news is that the Sun will never expand to the point where it
swallows the Earth. The bad news is that in about 1.1 billion years life
will be ‘cooked’ alive by a super greenhouse effect. This is the conclusion
of a group of American astronomers who have developed a model that sheds
new light on the ultimate fate of the Sun and Earth.

Astronomers have known for at least half a century that the Sun is growing
slowly more luminous and is now 30 per cent brighter than it was when it
formed 4.5 billion years ago. Eventually, it will become a bloated red giant.

Julianna Sackmann and Kathleen Kraemer of the California Institute of
Technology in Pasadena and Arnold Boothroyd of the University of Toronto
modelled the Sun’s evolution from its birth to the present, then extrapolated
into the future. Their model predicts that the Sun will brighten by 10 per
cent in 1.1 billion years. The warming that will result will bring Earth
to the threshold of a ‘moist’ greenhouse catastrophe, first predicted by
James Kasting of the University of Pennsylvania a few years ago.

Kasting’s model predicts that the higher temperature will raise water
vapour to the stratosphere. Water vapour is a greenhouse gas, which traps
heat, so the Earth will become hotter. But this heating will be partly
offset by another process. At the top of the atmosphere, ultraviolet from
the Sun will dissociate the water molecules into oxygen and hydrogen. The
hydrogen, being light, will escape from the atmosphere. The result will
be a gradual depletion of the Earth’s water.

Advertisement

Kasting’s model does not include global cloud cover, which probably
would offset some solar warming, and allow life to survive longer. However,
the model predicts that after the Sun becomes 40 per cent brighter than
it is today, about 3.5 billion years from now, the oceans will pump so much
water vapour into the atmosphere that the resulting greenhouse effect will
cause the oceans to evaporate completely. This will cause a ‘runaway greenhouse’
effect, with the Earth getting ever hotter. Ultimately, when the water vapour
is dissociated, the Earth will resemble modern Venus – extremely hot and
dry.

Sackmann and her colleagues agree with this scenario so far. But their
model diverges in 4.8 billion years, when the Sun exhausts its central supply
of hydrogen and becomes a red giant. The astronomers say that the Sun will
remain on the ‘main sequence’, burning hydrogen in a shell around its core,
until 6.4 billion years from now, when helium will ignite in its core. At
this time, the Sun will be about twice as bright as it is today. Over the
next 1.3 billion years it will slowly expand, transforming itself into a
red giant with a diameter 170 times its present one. This swollen Sun will
engulf the planet Mercury.

After a pause of about 110 million years, the Sun will begin to swell
rapidly for a period of 20 million years, until its surface reaches the
Earth’s present orbit. At this time, the Sun will be about 5200 times as
luminous as it is today.

What will save the Earth, says Sackmann, is the ageing Sun’s loss of
mass. The model she and her colleagues have developed is the first to consider
such an effect.

After expanding to engulf Mercury, the Sun will have only 72.5 per cent
of its present mass, while the expansion to the present Earth’s orbit will
see it left with only 59.1 per cent of today’s mass. As the Sun’s gravity
weakens, the planets will move outwards. Venus will drift out to 1.22 times
the Earth’s present distance, while the Earth will move to 1.69 times as
far away as it is today. This will make its orbit that of Mars today. However,
even at such a distance, the Earth will be heated to a temperature of 1600
K, at which point rocks melt.

The Sun will shed a little more mass as it contracts to a white dwarf,
letting Venus and Earth retreat to 1.34 and 1.85 times the Earth’s current
distance from the Sun. Both planets will then freeze as they orbit the shrunken
embers of the Sun.